A number of writers have described communications systems which employ pulse modulation in some form. Intelligence is conveyed in those systems by modulating pulse width or pulse amplitude, or the spacing between pulses, or some other characteristic. However, it appears that the prior art does not include a pulse modulation system which involves modulating pulse width by altering both the time of beginning and the time of ending of individual pulses relative to a standard time.
While the literature does include a description of a communication system which combines pulse modulation with time division multiplexing, it is the pulse rate that is modulated so that only one channel of information is transmitted. In a duplex system employing two stations, each with a receiver and transmitter, the pulse repetition rate of the received signal is used to control the unmodulated pulse repetition rate of the signal transmitted by that station. In such a system, the communications link can be considered to provide two channels of information, although one of them is limited to the transmission of synchronizing information. Further, such a scheme imposes the limitation that the band width of the intelligence transmitted by altering pulse repetition rate must be less than the pulse repetition rate itself.
Prior writers have shown that it is possible to provide a measure of security in the time division multiplexing of pulse rate modulated signals. However, the security aspect depends upon the relation between the pulse repetition rate and the propogation velocity of the signals. As a consequence, signals can be made secure only in the sense that communication is possible only between stations that are separated by a physical distance that corresponds to the pulse repetition rate employed in the communication.
It appears that the prior art does not include any modulation system in which pulse width modulation is accomplished by modulating both the beginning time and the ending time of the same pulse. Since that has not been done, the prior art does not include time-sharing, duplex communications that employ such a pulse width modulation scheme.